A threading device of a sewing machine inserts a thread into a thread hole of a looper which forms a seam by cooperating with a needle. In the threading device, there is known a technique in which an opening portion is provided on an outer side of a sewing machine frame, and compressed air is sprayed toward an inner part of a threading tube which is a path for a thread to pass through an inner side of the sewing machine frame, whereby the thread is fed toward the looper (see, e.g., Japanese Patent No. 2865470).
In this threading device, the air is compressed by an air pump interlockingly with a manual operation of a lever (see FIG. 5), and is discharged from a tip of a nozzle attached obliquely to a side surface of the threading tube, whereby the air is supplied toward the inner part of the threading tube.
The nozzle is obliquely attached at a position slightly on a downstream side from the opening portion of the threading tube so that the air is discharged from the tip at a position slightly on an inner side of the threading tube, i.e., the downstream side from the opening portion from which the thread is inserted. When inserting a tip end of the thread supplied from a thread supply source from the opening portion through the threading tube, the tip end of the thread is inserted until it reaches the downstream side of the tip of the nozzle, and the air is discharged by operating the lever, the thread is efficiently fed toward the downstream side of the thread path.
In the field of the sewing machine, moreover, there is known a technique to provide a transparent member on a thread path (see, e.g., JP 8-280965 A).
In the sewing machine disclosed in Japanese Patent No. 2865470, a thread inserting base which supports the threading tube is formed of an opaque member. Therefore, it is difficult for a user to visually check an amount of the thread inserted into a thread inserting port from the outer side of the sewing machine frame. Accordingly, in the event that the amount of the thread inserted into the thread inserting port is insufficient, there is a problem that the compressed air cannot be sprayed directly toward the thread so that the thread cannot be fed smoothly even if the lever for the air pump is operated repetitively. Consequently, there is a problem that the lever is uselessly operated many times, resulting in a decrease in working efficiency.
Even if the technique disclosed in the JP 8-280965 A is utilized to provide a transparent member for the thread path portion so that the thread inserting port is formed of the transparent member, there is a problem that, because threads to be used in the sewing machine are thin and have light colors such as white in many cases, it is difficult to see an inner side of the transparent member, resulting in a deterioration in workability.